1. Field of the Invention
This invention relates to zoom lenses, and more particularly to zoom lenses in which focusing is performed by using a part of a lens unit which moves axially in zooming.
2. Description of the Prior Art
In the prior art of zoom lenses, the so-called front focusing method has prevailed in which for focusing a front lens unit or one ahead of the zooming lens group is used. The zoom lenses having such focusing provision have the advantage that, regardless of zooming with changes in the focal length of the entire system, the focusing movement for the same object disatance, as measured from a position for an infinitely distant object, is always constant. However, this system has the following drawbacks:
(i) The weight of the focusing lens becomes heavy. PA1 (ii) The total focusing movement becomes extended. PA1 (iii) The range of variation of aberrations with focusing is usually widened. PA1 (iv) It is difficult t take the closest possible focusing object distance at a shorter value than ever before.
In general, improvements with respect to (iii) and (iv) worsen (i) and (ii), and it is particularly unsuitable to use as the optical system for auto-focus cameras.
To overcome such difficulties, it has been proposed that a lens unit which moves in zooming is used, or a lens unit that lies behind on an image side thereof is used to effect focusing. This method, when employed in a camera having an automatic focusing drive system, is advantageous since the loads on the motor and battery can be reduced. However, the use of the lens unit, which moves in zooming, for focusing often causes a problem in which due to the principles of design of the entire optical system, the refractive power of that lens does not have an adequate value so as to allow it to be used for focusing.
For example, in the cases where a variator lens for the image magnification, is used in focusing, since its refractive power is excessively strong, the focusing accuracy is decreased at wide angle positions. In the cases where a compensator lens is used, its refractive power is too weak to prevent the focusing speed from dropping when at telephoto positions.
Further, in this prior art focusing method, for one and the same object distance, the focusing unit has to be moved different distances to meet different focal lengths of the entire system. The distance the focusing unit moves from the position with an object at infinity varies with the focal length either in such a relation as in a curve of secondary degree, or in a discontinuous way. Then, as the object distance changes, the curve of secondary degree changes its shape. For this reason, an operating mechanism, which allows the same angle of rotation of the distance adjusting ring to effect focusing at all focal length positions over the entire zooming range, tends to be very complicated in structure, and is very difficult to realize economically. This tendency rapidly becomes significant as the zoom ratio increases.
Next taking an example of the conventional 4-unit zoon lens, the moving distances of the focusing unit for focusing by each lens unit are shown in FIG. 1. The zoom lens in FIG. 1 is specified to have the focal lengths f1, f2, f3 and f4 for the first to fourth units, respectively, along with the intervals l1, l2 and l3 between the successive two principal points of the units during zooming with an object at infinity as follows:
______________________________________ f.sub.1 f.sub.2 f.sub.3 f.sub.4 100 -40 111.167 121.273 f.sub. l.sub.1 l.sub.2 l.sub.3 80 10 44.5 15 144 36.67 23.17 9.67 200 46 4.5 19 ______________________________________
With the zoom lens of FIG. 1(a), where the second unit of negative power and the third unit of positive power are moved to effect zooming, when the first unit is selected for focusing purposes, as is widely accepted in the prior art, the focusing movement for one and the same object distance remains unchanged from a constant value at any station during zooming, as illustrated in FIG. 1(b). But when the focusing provision is made at one of the second to fourth units, it changes as illustrated in FIGS. 1(c), 1(d) and 1(e).
As zooming goes from the wide angle to the telephoto position, the second unit changes its function from a reducing system to an enlarging system past a point of unity of magnification. Since the second unit is of negative power, there results, therefore, in that part of the range of movement which provides the reducing system where the second unit has to be moved forward as focusing is effected down to shorter object distances, in the other part which provides the enlarging system, that it has to be moved rearward, and in the intermediate point for unity of magnification, that focusing becomes uncertain, a discontinuous focusing movement as illustrated in FIG. 1(c).
In the case of the third unit which is made also to serve for the focusing, since, in this conventional example, the zoom section (first to third units) is designed to be an afocal system, as the object distance decreases, the paraxial rays of light emerging from the third unit become divergent (at this time, this group serves as an enlarging system of positive sign). Therefore, it is only necessary to move the third unit rearward so that the diverging rays revert to parallel ones as illustrated in FIG. 1(d).
In the case of the fourth unit, the arriving rays are parallel to each other, and, therefore, it is only necessary to move the unit forward. However, since the afocal magnification increases or decreases with zooming, the value of the focusing movement differs with different focal lengths of the entire system as illustrated in FIG. 1(d). In this case, the distance the fourth lens unit moves to effect focusing is proportional to the square of the afocal magnification.
It should be also recognized that as the object approaches the lens (5m.fwdarw.2m), the total curvature of the curve of secondary degree becomes progressively more intensive.
In actual practice, therefore, to achieve focusing within the framework of the above-described zoom lens configuration by one of the second unit and those units that follow, it is required to make use of a three-dimensional cam or the like in the operating mechanism so that the curve of secondary degree, which represents variation of the focusing movement with zooming, varies in the total curvature with the object distance.
By fulfilling this requirement, the angle of rotation of the focusing ring can be made the same for one and the same object distance regardless of what focal length of the entire system may be. Otherwise, either means such as a computer circuit or the TTL auto-focus mechanism, must be provided for controlling the focusing operation electrically.
Apart from the above-described varieties of the focusing provision, it is also known in the art to provide a macrofocusing means by moving a lens member constituting part of a varifocal lens unit as disclosed in U.S. Pat. No. 3,661,445. As such, apart from the lens for focusing within the normal object distance range, in order to allow for focusing on a closeup subject, the varifocal lens unit after having been set in the telephoto position, as a whole is made movable in part.